One class of supported networks to which the present invention relates is frequently characterized by the term "printed circuits." (The term "circuit" will be used to signify one or more conductors, combinations thereof, electrical, including magnetic components per se, or such components and associated conductors.) The term "printed circuits" originates from the technique of printing the electrical assembly, which may comprise, for example, a network of conductors, the stator of a switch or the rotor of a motor, on an insulated base by means of the selective deposition of a conductive material thereon in conformity with the desired circuit configuration. Electrolytic, electroless and mechanical (spraying, sputtering, etc.) techniques have been employed to provide this printing operation.
In addition to the above, the term "printed circuits" has been applied to components, assemblies or circuits formed by techniques of selectively removing sections from an insulation-backed conductive blank, by selectively etching the non-conductive regions, the conductive components being protected by an etch resist printed on the blank in conformity with the desired circuit configuration.
In addition to the production of printed circuits, the methods of the invention are applicable to the production of other electrical components heretofore produced by solely mechanical means such as by stamping.
Burdening all of the foregoing techniques are certain limitations. Many of the techniques are not sufficiently accurate, require expensive machinery and are frequently impractical where a design is to be mounted on or laminated to an insulated base. The usual printed circuit techniques are environmentally undesirable and incompatible with the requirements for mass production, needing elaborate environmental control, having a susceptibility to latent defects in the resultant product (and thus requiring rigorous quality control), and being relatively expensive. Frequently it is necessary to provide temporary supports during various production stages. Moreover, the strength of many printed circuits leaves much to be desired. Generally, only relatively thin, flat structures can be produced. This, together with high resistivity and tendencies to delaminate and deteriorate under certain conditions have limited the applicability of these circuits. In spite of this, the trend is toward wider adoption of printed circuit techniques, this being due in part to the increasing emphasis on weight reduction and miniaturization and to the prohibitive costs in time, labor and materials of conventional circuit wiring and cabling procedures.
Developments in the electronics industry require the use of more densely packed electrical modules and circuits, each requiring multiple interconnections to one another. However, there is a practical limit to the density that can be achieved using conventional conductive networks. In a typical present day application, a floppy disk drive may require a connection to a recording head whose conductors are only on the order of 0.2 mm (0.008 inches) on center and associated jumper conductors must have a similar spacing. Further, recent liquid crystal displays have conductors which are even more closely packed, for example, 0.1 mm (0.004 inches) on center, with similar requirements for interconnection conductors. In addition, there is growing use of ceramic PC boards to accommodate multiple IC chip arrays which also require high density connectors and custom interconnect cables for purposes of terminating those components.
As a direct result of the growth in the circuit board industry, there has been a parallel increase in the volume of environmentally-hazardous chemicals generated by the conventional etching and deposition processes. For example, it is not uncommon for one circuit board fabrication facility to generate 4,000 liters (1,000 U.S. gallons) per day of photo resist stripper and 4,800 liters (1,200 U.S. gallons) per week of developer solution. These toxic wastes must be transported off-site for proper disposal at hazardous waste management sites. Thus, there is an urgent need for a practical non-chemical method for the manufacture of conductive networks, particularly high density conductive networks.
It is known to form a planar electrically conductive sheet into a non-planar pattern in a purely mechanical process, i.e. no etching is involved, by forming a conductive foil to define a circuit pattern spaced by waste material, the foil being attached to a dielectric material before, during or after forming, and surface machining the waste material off. However, the known methods of mechanically forming circuits have never been able to attain commercial acceptance due to technical problems and are unable to produce the high density networks required by modern technology.
One significant technical problem, ignored by the prior art, is what happens to the adhesive or dielectric when forming a planar electrically conductive sheet into a non-planar pattern. The known prior art suggests that the adhesive and/or dielectric is compressed into a smaller space. This is both dimensionally impractical and extremely unstable, as it builds compression stresses into the structure. As the waste material is machined off, the compressed material expands changing the location of conductors in the X, Y and Z planes thereby altering the desired circuit pattern, perhaps even causing portions of the desired circuits to be removed. This condition virtually eliminates any possibility of accomplishing the precision machining required to create fine line conductors in a high density network circuit. All known prior art ignores one or more of the following fundamental technical problems that have prevented these known processes from achieving any degree of commercial success. Current art does not teach us how to:
1. Form a sheet of conductive material as thin as or thinner than 0.02 mm (0.0007") thick sheet of conductive material into a non-planar pattern and protect its formed shape as it is processed through lamination and machining operations without damaging the formed structure; PA0 2. Form a laminate, consisting of a conductive material attached to a dielectric, while maintaining the flatness and precise location of conductors within the structure as is necessary to successfully remove all waste and maintain the desired conductor thickness; PA0 3. Form a laminate, consisting of a conductive material attached to a dielectric, while maintaining a substantially flat and stable reference plane necessary for precise location of conductors within the structure and for precise removal of waste material; PA0 4. Eliminate the distortion, resulting from compression stresses, that occurs as waste material flows away from the embossed pattern. NOTE: Material must be undistorted for the accurate grinding of waste material off; PA0 5. Stabilize, support and entrap a thin conductor (e.g. 0.02 mm (0.0007") thick and 0.025 mm (0.001") wide), to prevent its movement or delamination, as the waste material is mechanically ground off; PA0 6. Emboss a conductive foil into an adhesive layer less than half its thickness. (e.g. Embossing a 0.04 mm (0.0014") thick conductive foil (circuit pattern) into an adhesive layer less than 0.02 mm (0.0007") thick and surface machining off the waste material off leaving a 0.04 mm (0.0014") thick conductor; PA0 7. Create a finished circuit with a variety of conductor thicknesses designed to accommodate specific electrical and/or mechanical requirements; PA0 8. Eliminate the technical and cost limitations related to preparing and applying a dielectric overlay; PA0 9. Attach a temporary carrier that is generally a "compliant material" and as such can be conditioned (heat and/or pressure) to assists in first forming a structure (either a laminate consisting of a conductive material attached to a dielectric or conductive material) and once formed, maintaining that structure's critical dimensions (flatness of the temporary carrier necessary to establish and maintain a true machining reference plane based on the location of the conductors and the desired conductor thickness) necessary to successfully remove all waste material and maintain the desired conductor thickness. PA0 1) Forming, a planar conductive material into a non-planar pattern defining a desired circuit pattern; PA0 2) Positioning the formed conductors in a fixed spaced relationship to each other and to a reference plane, such as the planar exposed surface of a dielectric fast with the formed foil. This "reference plane" is critical to proper waste removal and conductor shaping (thickness, width and configuration) in high density conductive networks; PA0 3) Providing an adhesive and/or dielectric having a characteristics suitable to receive, capture and support a formed conductor pattern and controlling its flow during the forming operation. Use of an adhesive and/or dielectric in a state having the correct viscosity allows the elimination of half of a die set (either the male half when extruding a conductive material or the female half when forming a laminate); and PA0 4) Supporting each individual conductor, on at least three of its four sides, through the lamination and grinding operations. This support ensures that the conductors do not separate from the dielectric.
Due to the technical problems experienced by the prior art techniques they are unable to mass produce high density, multiple fine line circuit networks. The known techniques are limited to relatively thick, low density circuits and are therefore unsuited to meet today's demand for high density, fine line conductive circuits, for example multiple conductors spaced at 0.1 mm (0.004") on center.
It is a primary object of the present invention to provide a method of manufacturing a relatively inexpensive, high-quality, densely packed, supported conductive network for use in fabricating rigid or flexible circuit boards, without the use or generation of environmentally hazardous chemicals.
Other objects of the invention are to overcome shortcomings of the prior art as set forth in the numbered sub-paragraphs above and, in particular, to provide a method of forming a planar conductive material into a non-planar pattern either independently or when attached to a dielectric, in which:
a) the formed conductors may be positioned in spaced relationship to each other and to a fixed reference plane, defined by, for example, the dielectric, to ensure proper waste removal and conductor shaping, thickness, width and configuration);
b) a conditioned adhesive and/or dielectric is used to receive, capture and support a formed conductor pattern and to eliminate half of a die set (either the male, when extruding a conductive material or a female when forming a foil/dielectric laminate);
c) each individual conductor is supported on at least three of its four sides, through the lamination and machining operations to ensure that the conductors are not separated from the dielectric;
d) a flexible conductive network forms conductive paths which are profiled to self-align with corresponding conductors of other conductive networks;
e) a thin sheet of conductive material is formed into a non-planar pattern and processed without damaging the formed structure;
f) the flatness and precision, of a laminate, necessary to remove waste and maintain the desired conductor thickness is accurately maintained;
g) distortion problems that occur during the forming operations in the prior art are eliminated;
h) a method able to stabilize, support and entrap a thin conductor to prevent delamination as the waste material is ground off is provided;
i) a conductive foil is embossed into an adhesive layer less than half it's thickness.